The present invention is relating to a multi-chip flip-chip package, particularly to a face-to-face multi-chip flip-chip package.
An improved multi-chip flip-chip package had been disclosed in U.S. Pat. No. 6,084,308 entitled xe2x80x9cchip-on-chip integrated circuit package and method for making the samexe2x80x9d. A face-to-face flip chip assembly comprises a first chip and a second chip, and mounted on a package substrate. The package substrate has a cavity so as to accommodate a second chip which is face-to-face flip-chip mounted on the first chip. Also an insulation body is formed between the first chip and the second chip by capillary filling method. But a problem is that air pocket happens easily in the insulation body due to different capillary flowing speed. Please referring to FIG. 1, a common face-to-face multi-chip flip-chip package is disclosed. A second chip 30 that is smaller than a first chip 20 in size is flip-chip mounted onto the first chip 20, and furthermore the second chip 30 is placed in the opening 11 of the package substrate 10. When an underfilling material 40 is dispensed, the flow speed of the underfilling material 40 between square lateral walls of the opening 11 and the second chip 30 is much faster than that under active surface of the second chip 30 with the bumps under capillary effect. When the underfilling material 40 had flowed fast between the opening 11 and the second chip 30 and closed up the perimeters of the second chip 30, the underfilling material 40 among the bumps under the second chip 30 stop capillary flow to cause air pocket between the second chip 30 and the first chip 20 resulting in serious dispensing failure. Normally, the opening 11 of the common substrate 10 is rectangle so that the stress would be concentrated at the corners of the opening 11 to decrease the reliability of the product.
A primary object of the present invention is to provide a face-to-face multi-chip flip-chip package to solve the problem of harmful capillary flow of an underfilling material. The face-to-face multi-chip flip-chip package includes a package substrate having a concave wall to define a chip accommodation space. The concave wall is not parallel to an adjacent side surface of second chip inside the chip accommodation space. The concave wall is designed to reduce the capillary flow speed of underfilling material between the side surface of second chip and the chip accommodation space of the package substrate so as to avoid forming air pocket between second chip and first chip while dispensing underfilling material.
A secondary object of the present invention is to provide a face-to-face multi-chip flip-chip package. The face-to-face multi-chip flip-chip package includes a package substrate having a chip accommodation space defined by a concave wall for diminishing stress concentration of the package substrate.
According to the present invention, a face-to-face multi-chip flip-chip package comprises a package substrate having a top surface, a bottom surface and a concave wall between the top surface and the bottom surface. The concave wall defines a chip accommodation space, such as an opening or a cavity on the bottom surface. Preferably, the chip accommodation space is an opening in the shape of circular or elliptic shape to avoid the problem of stress concentration. A first chip, such as logic chip or other big size of chip, is mounted to the package substrate. The first chip has a first active surface with first bumps and a first back surface. The first chip is flip-chip mounted to the bottom surface of the package substrate. At least a second chip, such as a memory chip with a small size or other small size of chip, is flip-chip mounted to the first chip. The second chip has a second active surface, a second back surface and a plurality of side surfaces between the second active surface and the second back surface. Preferably, the second chip is disposed inside the chip accommodation space. By means of face-to-face flip-chip mounting technique, bumps are formed between the second active surface of the second chip and the first active surface of the first chip. An underfilling material is formed between the first chip and the second chip by dispensing method. The concave wall has a progressive distance (not parallel) from adjacent side surfaces of the second chip, that is the four corners of the second chip are closer to the concave wall to influence flow resistant of underfilling material, so that the capillary flow speed of the underfilling material between the side surfaces of the second chip and the concave wall will be reduced.